An optical fiber sensor comprises a measurement optical fiber whose optical characteristics are sensitive to a physical quantity. When light is injected into the optical fiber, a light signal is generated and detected by the sensor. Signal is then converted and processed to return the measured quantity. Optical fiber sensors are widely used in various types of applications, not only because of their small bulk (relatively small size and weight) and their insensitivity to electromagnetic disturbances, but also because they are particularly suited to the techniques of multiplexing and to the implementation of amplifiers or of distributed sensors. They also make it possible to limit the intrusive-ness of the sensor in the environment.
Some optical fiber sensors use Bragg gratings inscribed in the fiber. A Bragg grating is a reflector comprising alternate layers of different refractive indexes, which provokes a periodic variation of the effective refractive index in the optical fiber. The fiber sensors with Bragg grating are used to measure a physical quantity which corresponds to a constraint applied to the sensor. The constraint applied to the sensor induces a wavelength variation.
The optical fiber sensors with Bragg gratings can be passive or active (optical fiber laser sensor).
The optical fiber sensors with Bragg gratings are arranged in a protective enclosure, passed through from side to side by the optical fiber. When mounting such a sensor, it is useful to leave a relaxed additional length of fiber inside the enclosure. In effect, in the absence of such an additional length, the taut optical fiber risks generating a stiffness in a wide range of operation (temperature range in particular) which is prejudicial to the correct operation of the sensor. Moreover, such an additional length allows the fiber to filter the mechanical disturbances which can come from outside the sensor.
A known solution for producing an additional length of fiber is illustrated in FIG. 1. According to this approach, at least one loop 23 is made with the fiber 22 in the enclosure 20 which houses the sensor 21, which makes it possible to clear an additional length. However, such a solution generates a significant bulk because of the minimum bending radius allowed for an optical fiber (of the order of a cm). This solution is not therefore suited to compact sensors.